1. Field of the Invention
The present invention relates to a signal generator having a switching function for disconnecting an oscillator of a working system, when a failure occurs therein, and connecting an oscillator of a backup system.
2. Description of the Related Art
With recent advances in sophisticated information transmission, for example transmission of banking data, apparatus to be used for communication lines are required to have very high reliability.
Therefore, a method of improving reliability by providing a fully redundant structure and then switching into operation, as required, the redundant circuits has been provided for oscillators such as a local oscillator of a frequency converter in a radio communication device. However, if the switching time of the oscillator becomes longer, the oscillation signal disappears during the switching period and the frequency of a phase-locked loop circuit provided in the successive stage changes. Therefore, switching must be done within a very short period of time.
In a prior signal generator having a switching function, a watch-dog timer is provided as a failure detecting means of an oscillator and a switching instruction is issued to a switch upon judgement of a failure because no oscillation signal is input within a predetermined constant period.
An example of such a structure is shown in FIG. 1 and a timing chart indicating operations of each point A to F in FIG. 1 is shown in FIGS. 2A to 2F. In FIG. 1, the numerals 11 and 12 respectively denote crystal oscillators of the working system and backup system. A switch 14 selects any one output signal of these oscillators and inputs such output signal to a phase-locked loop (PLL) circuit 15. The PLL circuit 15 is provided for absorbing signal discontinuity during a switching operation by the switch 14. The numeral 13 denotes a control circuit such as a watch-dog timer for instructing switch 14 to perform the switching operation by detecting an OFF state of the oscillation signal A. The control circuit 13 is usually formed by a retriggerable one-shot multivibrator. Moreover, as the watching time for the detection of the OFF state, an R-C time constant is generally used.
However, in the prior art, a problem exists in that the switching operation is delayed for a period corresponding to the watching time. The watching time is required to have a duration based on the stability of the oscillation frequency and an error of the time constant (usually, 5 to 20% in the R-C circuit). Moreover the watching time is usually set to a period of 10 cycles or more in order to prevent switching from occurring frequently during the transitional period from the normal operating condition until perfect stoppage of oscillation. If switching from the oscillator 12 to the oscillator 11 is generated before the PLL circuit 15 is completely locked to a phase of the oscillator 12 after switching from the oscillator 11 to the oscillator 12, a phase error in the PLL circuit 15 is increased and the PLL circuit 15 may become out-of-synchronization.
Therefore, as shown in FIG. 2C, at least the time which is equal to several cycles of the oscillation signal may be necessary for the control circuit 13 to judge an OFF state of the oscillation signal A as a failure from the timing where the oscillation signal in the working system actually enters the OFF state (point X in FIG. 2A). Thereafter, the switch 14 switches the oscillator of the working system to that of the backup system as shown in FIG. 2D.
During the period T, which is from the timing of entering the OFF state to the timing of switching, an output frequency of PLL 15 changes by a large amount (.DELTA.f) as indicated in FIG. 2F because the switch 14 does not select the backup system even when the oscillation signal A is in the OFF state and the oscillation signal is not input to PLL 15 as shown in FIG. 2E.